The invention relates to a process and a device for contactless measurement of rotational speed with a proximity sensor operating according to the eddy current principle, where the proximity sensor comprises an inductor and a capacitor connected in parallel that form an oscillating circuit which can be excited from outside, and where the resonant frequency of the oscillating circuit experiences a detectable change when an object to be measured approaches the proximity sensor.
According to experience, the most advantageous measurement principle for contactless measurement of rotational speed of rapidly rotating turbines, especially of turbochargers, is the acquisition of data from each individual turbine blade by a sensor operating according to the eddy current principle. The advantages of a process of this type follow from, among other things, the fact that, for one thing, a measurement signal is present even at extremely low rotational speeds and, for another thing, the highest permissible rotational speed is theoretically unbounded. This is important since with modern turbochargers, depending on the size of the charger, rotational speeds of up to 400,000 revolutions per minute are to be expected. With that rotational speed, the circumferential speed at the outer diameter of the rotors is over 600 m/s. In addition, the measurement takes place almost without delay.
Additional advantages of a contactless measurement process according to the eddy current principle are due to the fact that the degree of efficiency of the turbocharger is not affected and only a minimum intervention in the geometry of the charger is required. In addition to this, the principle of measurement is not sensitive to dirt, no sealing problems occur, and the temperatures of −40° C. to 200° C. occurring on the compressor side of the turbocharger can be managed without difficulty.
Turbines in general as well as turbochargers in particular are, depending of the field of application, known in the most varied sizes. Charger sizes range in the normal case from a compressor wheel diameter of approximately 25 mm in small passenger vehicle motors to over 200 mm in commercial vehicles. Along with this, current blade thicknesses range from approximately 0.2 mm up to 2 mm. From these stated dimensions, the resulting detection distances between the individual blades are approximately 0.2 mm up to 2 mm.
In addition to the different geometries, dimensions, and rating of the existing turbochargers, differences in the material composition are to be taken into account. While in most turbochargers the blades are made of aluminum, titanium is coming into use to an increasing extent. In the normal case the sensor design must thus be adapted to the particular form of construction and to the particular material composition of the turbocharger. An inadequate adaptation of the sensor design to the particular circumstances of measurement can, under certain circumstances, lead to a sharp decrease in the precision of measurement.
The objective of the present invention is to specify a process and a device for contactless measurement of rotational speed of the type stated in the introduction with which a measurement of rotational speed is possible for a plurality of different geometries of the objects to be investigated and with sensitivity remaining nearly constant.